This invention relates to semiconductor devices and methods for their fabrication. More particularly, this invention relates to semiconductor devices having cured silicone covering the surface of the semiconductor elements. These coatings can be designed to resist swelling when cleaned by solvents such that the semiconductor elements and bonding wires of these devices are neither ruptured nor deformed by such cleaning. The coatings can also be designed to resist failure and faulty operation due to static electricity. This invention also relates to a method for fabricating these semiconductor devices.
The surface of semiconductor elements are typically coated with cured silicone to protect them from moisture and external stresses. This is true for both face-up semiconductor devices, in which the semiconductor element and lead frame are electrically connected through bonding wires, and for face-down semiconductor devices, in which the semiconductor element is electrically connected to the lead frame through solder bumps.
It has been found that using a solvent to clean the solder flux off these devices can cause problems. For instance, solvent cleaning can cause swelling of the silicone coating which results in warpage or destruction of the semiconductor element, deformation or breakage of the bonding wires and delamination of the solder bumps.
It has also been found that when such devices are used in a static-prone location, static electricity will gradually accumulate on the cured silicone covering the surface of the semiconductor element and either fail or operate erratically. This can occur, for instance, in applications such as image sensors used in facsimile equipment for reading the document to be transmitted or the thermal print heads used in printers for printing on the output paper.
To solve the solvent cleaning problem, the art suggests using (a) semiconductor devices in which the surface of the semiconductor element is coated with cured silicone and the element is then additionally sealed in a ceramic or plastic package and (b) semiconductor devices in which the surface of the element is covered with cured silicone containing a high loading of dispersed filler. The former solution results in increased costs and a reduced efficiency in mass production. The latter solution is disadvantageous in that heat cycling causes problems such as damage to the surface of the semiconductor element by the filler and deformation or rupture of the semiconductor element or bonding wires.
Similarly, to solve the static electricity problem, the art suggests using semiconductor devices in which the surface of the semiconductor element is coated with cured silicone and the element is then additionally coated with metal or plastic. Again, this solution results in increased costs and a reduced efficiency in mass production.
The present invention takes as its object the introduction of a semiconductor device whose semiconductor element is coated with a cured silicone which resists swelling when cleaned with a solvent. The semiconductor element and bonding wires are, therefore, neither ruptured nor deformed by solvent cleaning. An additional object of the present invention is the introduction of a semiconductor device whose semiconductor element is resistant to the failure and faulty operation which can be caused by static electricity. A final object of the present invention is the introduction of a method for fabricating semiconductor devices having these types of reliability.